Typical wireless communications systems, including cellular telephones, radios, and other wireless systems communicate data at high frequencies, i.e., at radio frequency (RF). Radio frequency signals are electrical signals conveying useful information having a frequency from about 3 kilohertz (kHz) to thousands of gigahertz (GHz), regardless of the medium through which such signals are conveyed. Thus an RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc. To process RF signals receive circuitry of a receiver, for example, generally converts the received RF signals to one or more lower frequencies, including an intermediate frequency (IF) and a baseband frequency. As an example, in a radio tuner, a frequency corresponding to a desired radio channel is tuned by mixing an incoming RF signal spectrum with a frequency generated in a local oscillator (LO) to obtain signal information of the desired channel. In various implementations, such a LO may be a voltage controlled oscillator or a numerically controlled oscillator (NCO), such as a digitally controlled oscillator (DCO).
In practice, a controlled oscillator can have its frequency controlled by changing capacitance values of one or more capacitors coupled to an oscillator element, such as a resonant tank. By varying the capacitance, the frequency generated by the controlled oscillator may be correspondingly varied. This capacitance may be controlled via digital control signals, e.g., a digital control word. Various implementations may include automatic frequency control (AFC) circuitry and/or algorithms to perform frequency control. Such circuitry and algorithms thus change the value of the digital signals that in turn change capacitance values, in turn adjusting the NCO frequency.
While such frequency adjustments are needed to tune to a desired channel, e.g., a radio station, a cellular frequency channel or the like, such frequency changes can have adverse effects on other parts of a receiver. For example, instead of adjusting a frequency continuously or smoothly, a NCO is discretely controlled, often leading to a step change in the digital control signals, which in turn leads to a corresponding frequency step change. Such a step change can induce noise in different receiver circuitry. For example, the step change can lead to generation of an undesired impulse in demodulation circuitry. Furthermore, the step change may appear as step change on the output of the demodulation circuitry, resulting in a step change in output audio signals, for example. Such step changes and impulse generation can undesirably affect receiver performance.
Accordingly, a need exists to reduce or eliminate such undesired effects of a frequency change.